Commonly-owned U.S. Pat. Nos. 6,503,786 and 6,664,594 to Klodzinski, incorporated herein by this reference, describe improvements in the manufacture and structure of silicon vertical power MOSFET devices to achieve increased SOA and to enhance linear operation of such devices. It is desirable to extend these capabilities to SiC vertical power MOSFET devices. However, the methods and structures employed in silicon power MOSFET technology do not readily extend to making SiC power MOSFET devices.
As shown in FIG. 1 (the ideal SOA of a Power MOSFET), the safe operating area of a Power MOSFET is limited by the blocking voltage on the right side, by the Rdson on the left side, by the maximum current rating and by the maximum power dissipation capability (the slanted lines on the upper right side of the SOA graph) of the device.
Recently, referring to FIG. 2, Spirito at al (FIG. 6 from Spirito, “Analytical model for thermal instability of low voltage power MOS and S.O.A. in pulse operation,” Proceedings ISPSD 2002, pp. 269-272) have shown that the SOA of silicon Power MOSFETs is in fact restricted on the high voltage high current side by the thermal instability of the device, with thermal instability triggered by the negative temperature coefficient of the Vth, if the device is operated at a drain current level below the Zero Temperature Coefficient Point.
A real SOA graph is shown in FIG. 3 (Actual SOA curves of a switching Power MOSFET) where it can be seen that both the bias conditions and the die temperature play a role in the thermal instability of the device.
As it is well known to the person familiar with the field, the On Resistance of the Power MOSFETs is lower if the density of the “cells” (the structure consisting of source, gates and source contacts) is higher.
As each cell is turned on, the slightest non-uniformity of the turn-on voltage from cell to cell will make one or several cells “steal” most if not all the drain current. This non-uniformity is normal in even state-of-the-art fabrication processing. Due to the negative temperature coefficient of the threshold voltage, the cells with increased current will have an even lower Vth and will start conducting even more current. The end result of such a local self heating phenomenon is the shorting of those cells. This effect, inherent to any MOSFET device, is very similar to the shorting of the base-emitter junction of a Power BJT due to the negative temperature coefficient of the Emitter Base diode.
In the case of a SiC MOSFET, for which a better thermal conductivity than silicon would seem to alleviate one aspect of this problem (the thermal one), the die size and the high packing density of the cell design aggravates the conditions that would initiate thermal instability under high bias conditions.
In addition, for a SiC Power MOSFET with a voltage rating of 1700V or lower, the channel resistance is the dominant component of the total ON resistance. Therefore, while in the saturation region, the temperature dependence of the channel resistance of a SiC MOSFET is of the utmost importance.
For applications where the Power Mosfet “operates” in the “saturation region” of the output characteristics an increased SOA of the device is significantly more important than its On Resistance and therefore trade offs to improve SOA at the expense of a higher Rdson are perfectly acceptable.
Power SiC transistors are commonly operated at high voltages and high drain currents, leading to considerable self heating, and in this way the operating temperature can be significantly higher.
Examples of applications where the “linear” operation of a Power Mosfet is needed are:                Battery charger (Cell phone, portable equipment, electrical vehicles)        Fan controller (automotive)        Power over Ethernet (TCP/IP routers, network switches)        Linear Power Amplifiers (audio)        Load switch, and virtually ALL applications where the device is switched ON-OFF and the “load” line travels through the high voltage-high current regimes of operation.In addition, a high frequency of operation will further degrade SOA, therefore all provisions from the U.S. application Ser. No. 13/195,632 are applicable to this patent.        